Stacked sheets and method of handling



Jan. 1l, 1966 J. J. MERCHAK STACKED SHEETS AND METHOD OF HANDLING Filed Sept. 20. 1962 INVENTOR.

United States Patent Office 3,228,7ll Patented Jan. 1l, 1966 3,228,711 STACKED SHEETS AND METHD F HANDLING Joseph J. Merchak, Hudson, Wis., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Sept. 20, 1962, Ser. No. 225,151 1 Claim. (Cl. 282-1) This invention relates to the assembling of paper or paper-like sheet materials, and particularly c-oated sheet materials, in stacks or bundles from which individual sheets are subsequently to be mechanically fed, as for eX- 'ample in printing or duplicating.

One application for which the invention has been found particularly advantageous is in the preparation on thin chemically coated masters or Systems A papers of printed originals from which copies are later to be made on cooperatively chemically coated Systems B copysheets by thermographic back-printing methods.

Thermographic back-printing .as here employed involves brief irradiation of an original having differentially radiation-absorptive image and background areas with intense radiation while the reverse surface of the original is in contact with the copy-sheet. A heat-pattern is induced in the more absorptive areas and the heat is conducted through the original to cause the formation of a corresponding visible copy on the copy-sheet. The original must -therefore be suiiiciently thin to permit heat transfer through the sheet without any significant heat diffusion laterally.

Papers of a suitable degree of thinness for thermographic back-print originals are unfortunately so flimsy or lacking in stiffness or rigidity as to present great difiiculty in handling during the preliminary printing of the graphic image. The situation is particularly ditiicult where the image is to be supplied by Xerographic printing methods, using machines such as the Xerox 914 electrostatic office copier. In this machine the coated master sheets are fed singly from a supply stack by a periodically rotated rubber-surfaced friction feed roller. The sheets are first printed with a fusible ink powder which must then be fused by passing the sheet through a heat-chamber. A series of feed rollers then removes the printed sheet from the machine. Thin -coated papers as used for Systems A thermographic master sheets, as well as other thin sheet materials, are difficult to feed to and through such printing machines. The single sheets are difficult to separate at the feeder. They are particularly susceptible to wrinkling and curling in the heating chamber, where any severe distortion may bring the combustible paper into contact with the nearby heating elements. They have a tendency to wrinkle while being drawn around the ejection rollers.

It has now been found possible to produce printed originals on thin papers, such as ther-mographic backprint Systems A master sheets, by Xerographic reproduction processes while avoiding the difficulties ordinarily encountered with thin papers in such processes, by suppying with the master sheets interleaved disposable reinforcing sheets which are preferably of low cost plain untreated paper. Surprisingly, it has been found that the reinforcing sheet will remain with the master sheet both in the feeding operation and throughout the entire printing, heating, and ejection sequence, without being adherently or otherwise attached thereto. Thereafter, the reinforcing sheet is easily removed and may be discarded, the printed master sheet being then used in the preparation of further copies on Systems B treated paper as previously described.

In the drawing, FIGURE 1 is a representation of apparatus which may be used in the selection of suitable reinforcing interleaf sheets, FIGURE 2 illustrates the simultaneous feeding of a master sheet and reinforcing sheet from a stack of sheets, and FIGURE 3 is a flow chart illustrating the process of stacking the sheets, printing the master sheet, and preparing it for further use.

Since the master sheet is to be printed on the chemically uncoated surface, it is necessary that the reinforcing or stiffening sheet remain associated with the opposite or chemically coated surface rather than with the uncoated surface as the sheets are fed from the stack. Many different sheet materials have been successfully tested in both positions. In some instances the nature of one or more coatings on one or the other or both of the sheets may be significant; or the microscopic contour of the sheet surfaces may have an effect. The composition of the paper or paper-like sheets may also be of importance. There may be other differences between particular sheet materials or surfaces thereof `which affect the ability of the reinforcing sheet t-o remain associated with the overlying master sheet during the feeding and printing operations while being easily separable from the underlying next adjacent master sheet in the stack of sheets. It has not been found possible, at least to the present time, to define adequately these several factors. However it has been found possible, by means of a simple test procedure, to determine accurately the ability of a particular combination -of sheets to meet the requirements hereinbefore set forth.

The test employed involves essentially the measurement of the angle of repose of a weighted sample of the one sheet on the surface of the other. Either sheet may be placed in either position with essentially no difference in the observed test value. The determination may be conveniently made by the use of apparatus indicated schematically in FIGURE l, by placing a first sheet 10, with its test surface uppermost, on the at surface of a horizontally disposed planar base member 11, clamping the -sheet to the base at one end by means of clamp 12, placing on the test surface a section 13 of the other test sheet with its test surface against the exposed test surface of the first sheet, temporarily anchoring the section by placing upon it a coextensive flat weight 14 which is prevented from slipping by being supplied on its lower or contacting fiat surface with an anti-skid compression pad 15, slowly raising the clamp end of the base member, as indicated by the arrow 16, the opposite edge 17 serving as a fulcrum and noting the angle with the horizontal, as shown on the sector 18 by the pointer 19, at which slippage of the weighted section on the clamped section first occurs. The anchoring weight is a thin flat block of steel plate having dimensions of 1A; x 3 X 7 inches (0.3 X 7.6 x 17.8 cm.). It carries a raised handle 20 on one surface for convenience in manipulating, and is provided over the other surface with a thin (1A inch, 0.6 cm.) sheet of sponge rubber 1S, the whole weighing 423 grams (about 0.93 pound). The test section and the anchoring weight are placed with the long dimension transverse of the direction of slip. The base member has a fiat polished steel surface on which the test sheet is supported, and the angle with the horizontal is uniformly increased over the pertinent range at a rate of one degree of arc per second. The weight is placed on the specimen and allowed to stand for Ione minute to assure equilibrium conditions before starting the angular movement of the supporting base. The angular position of the base at the start of slipping of the specimen is determinable to an accuracy of less than one degree of arc, and the test is reproducible at least to plus or minus one degree. The tests are conducted under condi-tions of constant temperature and humidity, i.e. lat 70 F. and 55 percent relative humidity.

Tests in which the angular movement of the base is intiated immediately after placing the samples in position are found to give somewhat erratic and usually somewhat 10W Master Sheet Reinforcing Sheet Master Sheet Reinforcing Sheet The master sheet, a typical System-s A master, employs as the paper base a sheet of light weight Ecusta Waylite offset paper having a basis weight of 30 pounds per 25 x 38/500 ream and provided with a high calcium carbonate loading on the felt side. The sheet is further coated on the wire side with a dry weight of A1.1 to 1.4 grams/ sq. ft. of a composition comprising a phenolic reducing agent for silver ion in a pigmented plasticized polymeric binder. The composition is ground in a ball mill to the extent necessary to produce a smooth non-streaky coating on the paper, backing. In the illustration the coating is represented by the row of asterisks. The reinforcing sheet is a thin Yankee finish machine glazed paper having a basis weight of 32 pounds per 24 x 36/500 ream, the unglazed surface being represented by the row of dashes; for purposes of identification, this sheet may for example be green in color.

When a stack of sheets as just described is subjected to the action of the properly adjusted feed mechanism of a Xerox 914 copying machine, or other suitable feeding mechanism, it is found that the sheets are fed in pairs or two-sheet composites as here indicated:`

Composite 4i.e. the smooth glazed surface of the glazed interleaf sheet slides over the uncoated surface of the next lower coated sheet while the rough unglazed surface of the interleaf and the coated surface of the outer coated sheet remain in mutual contact. The two-sheet composite has sufficient stiffness and body to pass through the copymachine without wrinkling or crumpling, and the two sheets remain together. After printing, the two sheets are easily separated.

The feeding action is illustrated in FIGURE 2 wherein a pair -of sheets It) and 13 is removed by a rotating feed drum 21 from a stack 22 of similar pairs supported on a spring-biased platform 23 and abutting against -a retainer plate 24. The two sheet-s and 13 remain together in congruency and slide easily from the next adjacent sheet 10.

A further and unexpected advantage provided by the reinforcing sheet during passage of the two thr-ough the copying machine is the elimination or substantial diminution of fumes. Since the phenolic component of the coating .on the coated sheet is volatilizable on heating, some of it is normally dispelled from the sheet during the heating cycle required for fusing the powdered fusible ink. The reinforcing sheet protects the coated surface and either inhibits volatilization or acts as a receptor for the portion vola-tilized.

The reinforcing sheet -of the combination just described is sufficiently inexpensive so that its disposal is of no great consequence. In an alternative system even this slight added expense is eliminated, by employing -additional master sheets as reinforcing sheets and then recovering such sheetsfor re-use. -matically illustrated as follows:

The sequence is sche- Master Sheet Inverted Master Sheet Inverted Master Sheet The two adjacent coated surfaces remain together; the two adjacent uncoated surfaces separate easily at the feeder.

Other paper-like sheet materials having sufficiently different opposite surfaces, either coated or uncoated, may similarly be fed in pairs from suitably prepared stacks and for a variety of purposes. In all cases the angle of repose between the two contacting inner surfaces of the pair of sheets must be at least two degrees but not more than about ten degrees of arc greater .than the angle of repose between the outer surface of the pair of sheets and the contacting -outer surface of lthe next adjacent pair.

At very low differences of repose angle it is normally possible to obtain essentially identical results in successive tests. Where this condition attains, stacks of composites having inner repose angles only two degrees greater than their outer repose angles have been found to feed successfully in well-adjusted friction feeders; whereas at lower values, or with sheet materials providing more erratic values, separation may occur at any interface indiscriminately.

With differences between the inner and outer repose angles of much more than about ten degrees of arc, resistance to separation of the several composites becomes so low as to make the stack or bundle difficult to handle. Papers coated on one surface with hard waxy materials and interleaved with uncoated papers provide an example of such materials. In stack form, these composites slide on each other so readily as to cause great difficulty in stacking, retaining, and transporting stacks of any significant number thereof.

The following examples of repose angles measured with various sheet materials will serve further to illustrate the practice of the invention.

Example I Systems A coated paper and Yankee finish reinforcing paper as hereinbefore defined are tested in various configurations, using the steel test block and movable base as described. With the coated surface of the Systems A paper against the glazed smooth surface of the interleaf, slipping occurs at 33 degrees. With the uncoated surface of the Systems A paper against the unglazed surface of the interleaf, slipping occurs at 27 degrees.

In another test, slipping between the coated surface of the Systems A paper and the rough surface of the interleaf occurs at 34 degrees; between the uncoated surface of the Systems A paper and the smooth surf-ace of the interleaf, at 27 degrees.

In either case the repose angle for the coated Systems A paper surface against one surface of the interleaf exceeds the repose angle for the uncoated Systems A paper surface against/the other surface of the interleaf by an angle of between two and about ten degrees. In either case the two-sheet composite of Systems A paper and interleaf is removed from the stack with the uncoated surface of the Systems A paper exposed, and the two sheets remain together as they pass through the printing machine.

' Example 2 Two sheets of Systems A coated paper, prepared as hereinbefore described, are tested for repose angle 1) in normal face-to-back position, (2) in face-to-faee position, and (3) in back-tchach position. The coated surface is here considered to be the face side.

Degrees Face-to-baek 37 Face-toface 33 Back-to-b-ack 30 Degrees Wire side to wire side 30 Felt side to felt side 3l Wire side to felt side 28 A stack of two-sheet composite-s of such paper in felt side to felt side, each composite thus being in wire-towire-side relationship with adjacent composite, fails to feed properly in the friction feed mechanism, showing that a differential repose angle of one degree is inadequate.

Example 3 Thin machine finish bond paper, i.e. a light Weight medium quality typewriter paper having a basis Weight of 13 lbs. per 17 X 22/500 ream, was tested for angle of repose as hereinbefore described, with results as follows:

Degrees Wire side to wire side 27 Felt side to felt side 20 Sheets of the paper were first stacked in conventional order, i.e. felt side of one against wire side of next adjacent, and an attempt made to feed separate sheets from the stack and through the xerographic copying machine. The light weight paper was found to feed erratically and to undergo curling and Wrinkling in the machine; and papers of less than about 20 pound basis weight are ordinarily not recommended for use in such copy machines. Another bundle of sheets was then stacked with alternate sheets in reverse position, forming a series of t\vo-sheet Composites with the sheets arranged wire side to wire side and the composites therefore arranged felt side to felt side. These composites were found to feed from the stack through the copying machine without any curling or wrinkling. The unprinted interleaf or reinforcing sheet of each composite was then easily removed from the printed sheet and could be re-stacked and re-run as desired.

What is claimed is as follows:

A method of avoiding distortion by wrinkling and curling of thin coated paper-like thermographic master sheets during printing and heating of said sheets in a Xerographic printing machine, said method comprising stacking said sheets congruently and in alternating position with reinforcing interleaf sheets selected and positioned as having with the coated surfaces of said master sheets Ian angle of repose, determined as described in the specification, of at least two and not more than about ten degrees of arc greater than their angle of repose with the uncoated surfaces of said master sheets; feeding a said master sheet with its uncoated surface in position for receiving a printed image by xerographic printing, together with a said interieaf sheet in contact with its coated surface, from said stack to said machine by applying lateral force at the exposed uncoated surface of the uppermost master sheet of said stack; permitting the couplet of master sheet and inter-leaf sheet to pass through said machine for printing 4and heating; and then separating said master sheet and said interleaf sheet.

References Cited by the Examiner UNITED STATES PATENTS 2/ 1951 Switzerland. 3/1951 France.

EUGENE R. CAPOZIO, Primary Examiner.

LAWRENCE CHARLES, JEROME SCHNALL,

Examiners. 

